1. Field of Invention
The present invention relates to a temperature control circuit and more particularly to a temperature control circuit which can respectively control two heating devices, such as electric blankets and hot packs, by using simple elements.
2. Related Art
Heating devices such as hot packs are very popular in the market. The heating of a heater wire is interrupted automatically after it is heated up to a temperature preset by the user to ensure safety, so that the heating temperature of the heating device can be maintained within a preset range so that the heating device can be used as a hot compress and its safety can be ensured.
In order to control the temperature effectively, U.S. Pat. No. 5,861,610 employs a positive temperature coefficient (PTC) element as the sensing wire to sense the changes in temperature, and a heater wire for heating up and temperature control. These techniques have already been disclosed in U.S. Pat. Nos. 6,300,597, 6,310,322 and 6,768,086.
U.S. Pat. No. 7,180,037 discloses an invention employing a positive temperature coefficient (PTC) element or a negative temperature coefficient (NTC) element, and the main differences between it and the above-mentioned conventional techniques lie in that: U.S. Pat. No. 7,180,037 senses the zero cross signal generated by the response of zero crossing of AC power signals, and senses the zero cross signal generated by the response of zero crossing of phase-shift AC power signals generated by the change of resistance caused by the change of temperature by the positive temperature coefficient (PTC) element or the negative temperature coefficient (NTC) element. By measuring the phase-shift time of the two zero cross signals and until that the phase-shift time is increased to reach the phase-shift time preset by a controller, a control signal is output by the controller to render the circuit connected or interrupted. As a result, both heating up and temperature control are achieved.
The above-mentioned temperature control methods can achieve the effect of controlling temperature. Nevertheless, the above-mentioned temperature control methods can only heat up the heater wire of one heating device. The below problems will occur when the heater wires of two heating devices are heated up respectively:
1. The manufacturing cost is increased because the heating up of the two heater wires requires two sets of temperature control circuits.
2. If one switch is used for controlling the two heater wires, the two heater wires can only be heating up at the same time or stopped heating up at the same time. The heater wire of each of the two heating devices cannot be controlled separately.
3. When one switch is used for controlling the two heater wires and the two heating devices are placed at different locations, different temperatures will be sensed by two sensing wires and the controller will use a highest temperature for temperature control. Therefore, when one of the heater wires has reached a preset temperature and is stopped being heated up continuously; the heating up of the other heater wire, which has not reached a preset temperature, is also interrupted. As a result, the hot compress function of the two heating devices can not be used at the same time which is inconvenient for using.
In view of the above problems, a temperature control circuit of the present invention is disclosed to control two heating devices separately and can also save the manufacturing cost.